Sheet glass core drilling machine

ABSTRACT

A machine for drilling through sheets of glass or glass-like material with a pair of opposed cooperating coaxial drilling tools located on opposite sides of the sheet. The tools are alternatingly fed into the sheet to a depth of about half the thickness of the sheet by means of fluid cylinders with pistons connected to the respective drilling tools. The fluid operating system for the cylinders drives each of the pistons at a constant feed rate during both a fast feed interval and a slow feed interval and adjustable timers are used in association with the fluid control system to afford precision control of the depth of cut.

United States Patent 11 1 Highberg et a1.

[ SHEET GLASS CORE DRILLING MACHINE [75] Inventors: Carle W. l-llighberg; George R. Roesch, both of Sylvania, Ohio [73] Assignee: Englehard Mineral & Chemical Corporation, Murray Hill, NJ.

Field of Search... 51/109 R, 111 R, 125, 80 R, 51/81, 165.77, 1659; 408/37, 130; 175/330,

[561 References Cited UNITED STATES PATENTS 2,860,487 11/1958 Wheeler 408/130 X 2.941.338 6/1960 Santschi....

3,124,016 3/1964 Reaser 408/37 [11] 3,813,820 June 4, 1974 3,401,583 9/1968 Jacobson et a1. 408/130 Primary Examiner-Othell M. Simpson 1 ABSTRACT A machine for drilling through sheets of glass or glasslike material with a pair of opposed cooperating coax ial drilling tools located on opposite sides of the sheet. The tools are alternatingly fed into the sheet to a depth of about half the thickness of the sheet by means of fluid cylinders with pistons connected to the respective drilling tools. The fluid operating system for the cylinders drives each of the pistons at a constant feed rate during both a fast feed interval and a slow feed interval and adjustable timers are used in association with the fluid control system to afford precision control of the depth of cut.

10 Claims, 10 Drawing Figures PATENTEUJUH 4 I974 SHEET 1 0F 5 FIG.

PATENTEDJUN 4 is];

PATENTEDJUH 4 I974 SHEU 3 [If 5 1 SHEET GLASS CORE DRILLING MACHINE BACKGROUND OF THE INVENTION This invention relates to the drilling of sheets of glass and glass-like material using, for example, diamond grit-type core drills and the like. More particularly, the invention relates to a fluid operating system for advancing and retracting a pair of opposed coaxial drilling tools through their respective alternating operating cycles and especially to the control of the fluid operating system to precisely gauge the depth of cut. The invention has particular utility in connection with the drilling of holes in automobile door lites as is required in the case of door window glass for many current US. makes of automobiles, as well as in the case of lites for patio doors and the like.

In drilling holes in sheet glass and other hard refractories it is necessary to use extremely hard cutting materials. Diamond grit-type core drills (industrial diamond grit embedded in an annular matrix commonly formed of fused powdered metal) are most commonly used for this purpose. In order to avoid spalling or fracturing of the glass as would normally occur around the margin of the hole in the glass surface where the drill would normally exit, it is more advisable to drill the hole partway through from one side of the sheet and the balance of the way from the other side to remove the core. When this procedure is followed, the final breakout occurs between the surfaces of the glass and spalling of the glass surface is avoided.

Normally two cooperating coaxial core drills are used, the drills being advanced into the glass alternatin gly from opposite sides of the glass sheet. The glass sheet is firmly clamped in the machine and cooling water is supplied to the region around the area to be cut. It is necessary that precise control of the core drill travel or depth of cut be achieved in order to assure that the drilling from at least one side stops at the proper depth.

Prior art machines for the purpose described have conventionally utilized a constant pressure operating system (e.g., a fluid pressure system) to advance the drills into the glass. The respective drills are conventionally connected to the operating rod of the fluid cylinder and then reciprocated by the cylinder and piston through an operating cycle. The depth of cut is conventionally controlled by mechanical stops and electrical switches operatively connected to solenoid-actuated valves in the control system.

This arrangement requires time-consuming manual adjustment when adjustment is needed and also tedious readjustment to set up a machine to drill glass sheets of a different thickness. The positioning of the switch actuators or stops must be quite precise and has in the past required considerable time for adjustment and readjustment in order to obtain satisfactory results. Also the cooling water supplied to the cutting area splashes around the lower part of the machine andfrequently shorts out the electrical switches, causes corrosion of the contacts and otherwise renders the switches unreliable.

In many circumstances the type of glass sheet to be drilled (and thus the thickness) is changed frequently and with each change the switch mechanisms must be manually positioned and readjusted by trial and error in order to set the depth of cut for the dimensions of the particular type of glass sheet. Furthermore, as the drill bit wears, the switch mechanisms and/or stops must be periodically readjusted to maintain the desired depth of cut. 1

Another approach to sheet glass core drilling, as concerns the operating system for reciprocating the drills through their respective operating cycles, is to use a constant feed rate in advancing the drills rather than a constant pressure. Whereas in constant pressure drilling the cutting progresses at a rate depending upon various factors such as the physical characteristics of the material being drilled, in constant feed drilling an excess pressure is used but the cutting proceeds at a uniform rate regardless of the load encountered by the drill, assuming, of course, that the pressure is sufficient to overcome all loading conditions encountered.

The advantages of constant feed rate core drilling include longer drill bit life and adecrease in the required frequency of sharpening. A general discussion of constant feed rate drilling is contained in an article entitled An Improved Method of Diamond Core Drilling Automotive Glass in the periodical publication Industrial Diamond Review of May 1969. A further discussion of constant feed rate drilling is contained in US. Pat. No. 3,710,516 of Joseph B. Kelly. That patent contains a detailed discussion of the advantages achieved by advancing the core drill at a constant speed.

The present invention provides an improvement relating to the control of a fluid operating system for a machine of the general type disclosed in the above reference patent, and affords other features and advantages heretofore not obtainable.

SUMMARY OF THE INVENTION lt is among the objects of the invention to control with increased precision, the depth of cut of a drill advanced into a sheet of glass or glass-like material at a constant rate of speed.

Another object is to reduce the set-up time required to adjust a machine for drilling sheets of glass and other glass-like material to accommodate sheets of different dimensions.

Still another object is to eliminate electrical switches for controlling the feeding of drills into sheetsof glass andglass-like material.

A further object is to reduce the duration of a glass sheet core drilling operation by advancing the drill at a relatively fast feed rate up to a position close to the surface of the glass and then at a relatively slow feed rate through the glass to a desired depth of cut.

These and other objects are accomplished by means of the novel fluid pressure system embodying the unique control means of the invention as used in association with a sheet glass core drilling machine of the type described. For each of the two opposed coaxial drilling tools of the machine there is provided, in accordance with the invention, a fluid cylinder with a piston operatively connected to the tool. Each tool is extended and retracted by a fluid pressure system operatively connected to the respective cylinder to extend the drilling tool at a constant feed rate time interval of fast travel up to a position closely spaced from the surface of the sheet to be drilled and at a constant feed rate interval of slow travel from engagement of the drilling tool with the surface of the sheet to a predetermined depth of cut. The distance of travel during each interval is precisely controlled by adjustable timing means operatively connected to the fluid pressure sys- 3 tem for controlling the duration of the constant feed rate time intervals. Thus, the timing means serves to control the positions of the drilling tool at the initiation and termination of the slow extension time interval to control the depth of cut into the glass sheet.

The timing means most advantageously includes a separate electronic timer for each of the time intervals. Even more advantageously, each of the timers is a digital-type timer which is capable of adjustment in very small time increments to afford optimum precision in control and adjustment of the depth of cut, and especially to adjust for progressive wear of the tool bit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sideelevation of a sheet glass core drilling machine embodying the invention'with parts broken away and shown in section for the purpose of illustration;

FIG. 2 is a front elevation of the machine of FIG. ll;

FIG. 3 is a fragmentary sectional view on an enlarged scale taken on the line 33 of FIG. 2;

FIG. 4 is a fragmentary sectional view on the line 4-4 of FIG. 3;

FIG. 5 is a fragmentary front elevational view taken from the line 5-5 of FIG. 2;

FIG. 6 is a sectional view on an enlarged scale taken on the line 6-6 of FIG. 1;

FIG. 7 is a sectional view on an enlarged scale taken on the line 77 of FIG. I; and

FIGS. 8, 9 and 10 are schematic diagrams illustrating sequentially the fluid pressure system and associated control mechanisms of the invention and the conditions thereof during sequential stages in an operating cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings and initially to FIGS. 1 and 2, there is shown a sheet glass core drilling machine with a fluid pressure operating system utilizing a control means embodying the invention. The machine has an upper spindle assembly A and a lower spindle assembly B adapted for vertical travel alternatingly toward and away from one another through a core drilling cycle. The machine receives a horizontal glass sheet which is held during the drilling operation by a clamp assembly C. In the particular embodiment shown, the machine is adapted to drill at a predetermined location, a hole in a glass sheet 10 cut to a desired contour for an automobile door lite.

General Arrangement The machine is mounted on a granite toe plate 11 which normally rests against a granite surface plate. The location of the toe plate is readily adjustable by means of a water pressure system whereby water is forced into a shallow recess in the bottom surface of the toe plate through vertical passages to counteract the weight of the machine and facilitate adjusting the location of the machine on the surface plate. Water is subsequently removed from the recess and a vacuum applied through the same vertical passages to anchor the machine in the desired location. A supporting frame 12 is bolted to the toe plate 11. v

The upper and lower spindle assemblies A and B are supported for vertical movement alternatingly toward and away from one another in upper and'lower dovetail guides 13 and 14 respectively secured to the frame 12. The advancing and retracting of the spindle assemblies A and B is accomplished by means of hydraulic drive cylinders 15 and 16 respectively mounted at the top of the supporting frame 12 and powered by a fluid pressure system best illustrated in FIGS. 8, 9 and 10. The cylinders 15 and 16 are both located at the upper portion of the machine to provide a location as remote as possible from the splash of water used to lubricate the drills.

The piston rod for the drive cylinder 15 is connected directly to the upper spindle assembly B while the piston rod for the drive cylinder 16 is connected to a vertical connecting rod 17 pivotally connected at its lower end to one end ofa rocker arm 18. The arm 18 is connected at its mid portion to the frame 12 for pivotal movement about a horizontal axis. The other end of the rocker arm 18 is connected to a vertical connecting rod 19 which in turn is pivotally connected at its upper end to the lower spindle assembly B (FIG. I).

The clamp assembly C best illustrated in FIGS. 3, 4 and 5 includes a horizontal table 20 mounted on the frame 12 and a clamping lever 21. The table 20 is adapted to'support the glass sheet 10 and has a hole 22 for the drill of the lower spindle assembly B. The clamping lever 21 is located above the table 20 and is pivotally connected to brackets 23 on the supporting frame 12. A'clamping jaw 24 with a hole 25 to accommodate the drill of the upper spindle assembly A is pivotally connected to the bifurcated outer end of the clamping lever 21. The clamping jaw 24 is moved by the lever 21 toward and away from the table 20 to grip and hold the glass sheet 10 in the marginal zone surrounding the location of the hole to be drilled; The clamping lever 21 is pivotally connected at its opposite or inner end to a connecting rod 26 extending from the piston of a clamping cylinder 27.

Spindle Assemblies The spindle assemblies A and B are essentially identical to one another and thus will be described herein only with respect to the upper spindle assembly A. Like numerals will be applied to corresponding parts of the lower spindle assembly B.

The assembly A comprises a platform 31 with a slide 32 welded to its outer end (FIGS. 1, 2 and 6). The slide is guided for vertical movement in the dovetail guide 13 and is reciprocated by the drive cylinder 15. The piston of the cylinder 15 is connected to a link that in turn is pivotally connected to the top of the slide 32. The slide 32 supports a spindle 33 with a chuck 34 that receives a core drill 35; The upper end of thespindle 33 has a pulley 36 driven by a V-belt 37 extending from the drive pulley 38 of an electric motor 39. The motor is adjustably mounted on the platform 31 in the manner shown, and is adapted to turn the core drill35 at about 3,100 RPM.

In order to compensate for the weight of the motor 39, an air balance cylinder 40 is provided at the rearward end of the platform 31. Water for lubricating the core drill 35 is supplied through a flexible water conduit (not shown) connected to a rotary coupling which in turn is connected to the spindle 33.

Fluid Pressure System The fluid pressure operating system for the drive cylinders l5 and 16 and the control mechanisms associated therewith are best seen in FIGS. 8, 9 and 10. The systems are identical with respect to the upper and lower spindle assemblies A and B and therefore will be described only with respect to the upper spindle assembly A and its associated drive cylinder 15;

Fluid pressure is obtained by a constant pressure pump 50 that draws from a reservoir 51 and supplies operating fluid under pressure through a line 52 to the head end of the drive cylinder 15. The supply of pressure through the line 52 is controlled by a four-way solenoid valve 53 shown in its piston extending position in FIGS. 8 and 9 and in its piston retracting position in FIG. 10.

Fluid is supplied and exhausted from the rod end of the drive cylinder 15 through a line 54 with a pair of parallel branches 55 and 56. The branch 56 has a twoway solenoid valve 57 shown in its open position in FIGS. 8 and and in its closed position in FIG. 9. The branch 56 also has a ball-type check valve 58 and a cross connecting line 59 connecting it to the branch 55 between the check valve 58 and the two-way valve 57.

The branch 55 has a pressureand temperaturecompensated, adjustable flow restricter 60 (or flow control) located between the drive cylinder and the cross connecting line 59 and another similar adjustable flow restricter 61 located between the four-way valve 53 and the cross connecting line 59. The two-way valve 57 is in its open position as illustrated in FIGS. 8 and 10. As viewed in FIG. 8, fluid being exhausted from the rod end of the cylinder 15 passes through the two-way valve 57 bypassing the flow restricter 60 and then flows through the flow restricter 61 since the check valve 58 will be in the closed position. This condition provides for relatively fast extension at a constant feed rate. When the two-way valve 57 is in its closed position as illustrated in FIG. 9, fluid being exhausted from the rod end of the drive cylinder 15 will be directed through both the flow restricters 60 and 61. This condition provides for relatively slow extension at a constant feed rate (e.g.. 0.06 in. per sec.).

When the four-way valve 53 is in its retracted'position illustrated in FIG. 10, fluid pressure passes through the line 54 to the rod end of the drive cylinder 15. In this condition the two-way valve 57 is in its open position and the flow passes through the branch 56 since the check valve 58 is in its open position thus enabling the fluid pressure supplied to bypass both of the flow restricters 60 and 61. v v

The solenoid 53a for, the four-way valve 53 and the solenoid 57a for the two-way valve 57 are energized by' electronic timers 65 and 66 respectively. In the particular embodiment of the invention described herein, the timers 65 and 66 are solid-state electronic timers with digital read-out and pushbutton adjustment of the type sold by Automatic Timing and Controls, Inc. of 203 S. Gulph Rd., King of Prussia, Pa.. under the trade designation 335 A35 lAl OPX. These timers are adjustable in increments of and the digital read-out permits quick push-button setup and adjustment by a machine operator.

Operation The operation of the control mechanism associated with the fluid pressure operating system for the machine described and shown herein will'be discussed with reference to FIGS. 8, 9 and 10 which illustrate sequentially an operating cycle for the upper spindle assembly A. As indicated above, the systems are essentially identical with respect to the upper spindle assembly A and lower spindle assembly B and will be described with reference to the upper spindle assembly A only.

Referring to FIG. 8, there is shown the initial portion of the operating cycle wherein the upper spindle assembly A is to be advanced at relatively fast travel at a constant speed rate up to a position closely spaced from (e.g., one thirty-second inch from) the surface of the glass sheet 10 but not into engagement with the glass. It is desirable that this portion of the cycle provide an interval of fast travel to minimize the duration of the drilling operation. The system in this condition has been activated by an operator and the timer switch 65a is closed to energize the solenoid 53a so that the fourway valve 53 is in its normal position or moved to the right as viewed in the drawings. Accordingly, fluid pressure is supplied to the head end of the drive cylinder 15 through the line 52. The timer 65 is energized and is timing out a predetermined time interval.

The timer switch 66 is open, the solenoid 57a is deenergized, and the two-way valve 57 is in its open position so that fluid being exhausted from the rod end of the drive cylinder 15 bypasses the restricter and is checked only by the flow restricter 61. Since only one of the two restricters is connected in the exhaust line, a constant feed rate is provided but the rate of piston travel is relatively fast. The timer 66 is energized and the interval of fast travel continues until the timer 66 times out to close timer switch 66a and deenergize solenoid 57a. Thus, the interval of fast travel is determined by the setting of the timer 66.

Referring next to FIG. 9, it will be seen that when the timer 66 times out the solenoid 57a is energized and the valve 57 moves to its closed position so that the fluid exhausted from the rod end of the cylinder 15 is directed through the flow restricter 60 as well as the flow restricter 61 to provide an increased flow check. In this condition a slow rate of travel is provided for the upper spindle assembly A and the time interval continues as long as pressure is applied to the head end of the cylinder 15. During this time interval the drill 35 is moving into engagement with or cutting into the glass and proceeding at a constant feed rate to a predetermined depth of cut. This time interval of slow travel at constant feed rate continues until the timer times out and deenergizes the solenoid 53a. Assuming the flow restricter 60 is adjusted to provide a slow travel constant feed rate of 0.06 in. per sec., and the timer 65 is adjustable in increments of 0.01 sec., the depth of cut may be pre-adjusted in increments of 0.006 inch.

Referring next to FIG. 10, which illustrates the condition of the system during retraction of the upper spindle assembly A, it will be seen that the timer 65 has timed out and the switch 650 has opened to deenergize the solenoid 53a. Consequently, the four-way valve 53 is moved to the left or to its reverse position. In this position pressure is supplied by the pump 50 through the line 54 to the rod end of the drive cylinder 15. Both flow restricters 60 and 61 are bypassed since the branch 56 is open through the check valve 58 and the two-way solenoid valve 57. The retraction will proceed until the upper spindle assembly A is fully retracted to the starting position shown in FIG. 8.

The lower spindle assembly B is preferably cycled before the upper spindle assembly A and its cycle is iden tical to the cycle described above with respect to the fluid pressure system for the upper spindle assembly A.

After the second cycle of the upper spindle assembly A is completed, the cutting operation of both drills will have been accomplished and the core will be ejected. Finally, the clamp assembly C will be released and the glass sheet 10 removed from the machine by the operator.

The releasing of the glass by the clamp assembly C is preferably controlled by a sequencing timer (not shown) which is energized by the operator when he pushes a start button. The start button also energizes a solenoid valve that actuates the clamping cylinder 27 to cause the clamping jaw 24 to clamp the glass sheet against the table 20. The initial clamping pressure is of a relatively high order, sufficient to counteract any force applied against the glass by the core drill 35 of the lower spindle assembly B as it advances upwardly into the glass from below. After the lower spindle assembly B has completed its cutting, the clamping pressure applied through the clamping cylinder 27 may be reduced to a nominal level in response to the sequencing timer since the force applied against the glass by the core drill 35 of the upper spindle assembly A is counteracted by the table which is rigidly supported by the frame.

. With the control mechanism of the invention, the depth of cut can be very accurately controlled (e.g., in increments of 0.006 inch) and uniformly repeated during each cycle. Also the point of initiation of the slow feed interval can be precisely determined according to the glass thickness using the timer 66.

To set up the machine for the initial drilling operation with a certain glass thickness the operator can adjust the depth of cut by trial and error if desired by merely punching the pushbuttons on the respective timer to increase or decrease a prior digital read-out in increments of 0.0l sec. As indicated above, this translates into depth increments of approximately 0.006 inch where the feed rate is 0.06 in. per sec. The slow travel feed rate can be varied by adjusting the flow restricters 60 and 61. The speeds can be varied, for example, between limits of 0.02 in. per sec. to 0.5 in. sec. within tolerances of 10.001 in. The increments of time adjustment (e.g., 0.01 inch) thus translate directly into distance increments depending on the speed rate.

Once the setup has determined the proper timer settings for a particular production run, he need merely record the settings so that the next time an identical run is made he can immediately and quickly preset the timers 65 and 66. He must, however, take into account drill bit wear.

While the invention has been shown and described with respect to a specific embodiment thereof, this is intended for the purpose of illustration rather than limitation and other modifications and variations of the specific machine herein shown and described will be apparent to those skilled in the art all within the intended scope and spirit of the invention. Accordingly, the patent is not to be limited to the specific embodiment herein shown and described nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced by the invention.

We claim:

1. Apparatus for driving a drilling tool through an operating cycle of reciprocating travel including an interval of drilling into a sheet of glass or glass-like material, comprising:

a fluid cylinder, a piston in said fluid cylinder operatively connecte to said drilling tool, fluid pressure means operatively connected to said fluid cylinder to extend said drilling tool at a constant feed rate interval of fast travel up to a position close to the surface of said sheet and a constant feed rate interval of slow travel from engagement of said drilling tool with the surface of said sheet to a predetermined depth of cut into said sheet, and

adjustable timing means operatively connected to said fluid pressure means for controlling the duration of said constant feed rate time intervals whereby to control the positions of said drilling tool at the initiation and termination of said slow extension time interval to control the depth of cut into said sheet.

2. Apparatus as defined in claim 1 wherein said fluid pressure means includes a first flow restricter and a second flow restricter and valve means for selectively connecting only one of said restricters in flow restricting relation to said drive cylinder to control fluid flow rate and provide said fast travel and for selectively connecting both of said restricters in flow restricting relation to said drive cylinder to control fluid flow rate and provide said slow travel.

3. Apparatus as defined in claim 2 wherein said valve means comprises two solenoid-operated valves.

4. Apparatus as defined in claim 3 wherein said solenoids are energized by said timing means.

5. Apparatus as defined in claim 1 wherein said adjustable timing means comprises two electronic timers.

6. Apparatus as defined in claim 5 wherein said timers have digital read-out.

7. Apparatus for drilling holes in a sheet of glass or glass-like material with a pair of opposed cooperating coaxial drilling tools located on opposite sides of the sheet and adapted to be fed alternatingly into said sheet about one-half the depth thereof comprising:

means for driving said tools through alternating operating cycles of reciprocating travel, each cycle including an interval of drilling into said sheet, said means including for each drilling tool;

a fluid cylinder,

a piston in said fluid cylinder operatively connected to said drilling tool,

fluid control means operatively connected to said fluid cylinder to extend said drilling tool at a'constant feed rate time interval of fast travel up to a position close to the surface of said sheet and a constant feed rate time interval of slow travel from engagement of said drilling tool with the surface of said sheet to a predetermined depth of cut into said sheet, and

adjustable timing means operatively connected to said fluid control means for controlling the duration of said constant feed rate time intervals whereby to control the positions of said drilling tool at the initiation and termination of said slow extension time interval to control the depth of cut into said sheet.

8. Apparatus as defined in claim 7 wherein the axis of said drilling tool is vertical and wherein said tools 10 gripping pressure to said glass sheet.

10. Apparatus as defined in claim 9 wherein said fluid pressure means includes control means for applying a relatively high clamping during the drilling operation of said lower drilling tool and a relatively low clamping pressure during the drilling operation of said upper dril- 

1. Apparatus for driving a drilling tool through an operating cycle of reciprocating travel including an interval of drilling into a sheet of glass or glass-like material, comprising: a fluid cylinder, a piston in said fluid cylinder operatively connected to said drilling tool, fluid pressure means operatively connected to said fluid cylinder to extend said drilling tool at a constant feed rate interval of fast travel up to a position close to the surface of said sheet and a constant feed rate interval of slow travel from engagement of said drilling tool with the surface of said sheet to a predetermined depth of cut into said sheet, and adjustable timing means operatively connected to said fluid pressure means for controlling the duration of said constant feed rate time intervals whereby to control the positions of said drilling tool at the initiation and termination of said slow extension time interval to control the depth of cut into said sheet.
 2. Apparatus as defined in claim 1 wherein said fluid pressure means includes a first flow restricter and a second flow restricter and valve means for selectively connecting only one of said restricters in flow restricting relation to said drive cylinder to control fluid flow rate and provide said fast travel and for selectively connecting both of said restricters in flow restricting relation to said drive cylinder to control fluid flow rate and provide said slow travel.
 3. Apparatus as defined in claim 2 wherein said valve means comprises two solenoid-operated valves.
 4. Apparatus as defined in claim 3 wherein said solenoids are energized by said timing means.
 5. Apparatus as defined in claim 1 wherein said adjustable timing means comprises two electronic timers.
 6. Apparatus as defined in claim 5 wherein said timers have digital read-out.
 7. Apparatus for drilling holes in a sheet of glass or glass-like material with a pair of opposed cooperating coaxial drilling tools located on opposite sides of the sheet and adapted to be fed alternatingly into said sheet about one-half the depth thereof comprising: means for driving said tools through alternating operating cycles of reciprocating travel, each cycle including an interval of drilling into said sheet, said means including for each drilling tool; a fluid cylinder, a piston in said fluid cylinder operatively connected to said drilling tool, fluid control means operatively connected to said fluid cylinder to extend said drilling tool at a constant feed rate time interval of fast travel up to a position close to the surface of said sheet and a constant feed rate time interval of slow travel from engagement of said drilling tool with the surface of said sheet to a predetermined depth of cut into said sheet, and adjustable timing means operatively connected to said fluid control means for controlling the duration of said constant feed rate time intervals whereby to control the positions of said drilling tool at the initiation and termination of said slow extension time interval to control the depth of cut into said sheet.
 8. Apparatus as defined in claim 7 wherein the axis of said drilling tool is vertical and wherein said tools comprise an upper drilling tool and a lower drilling tool.
 9. Apparatus as defined in claim 8 including means for clamping said glass sheet in a horizontal position interposed between said tools, said means comprising a horizontal table, a clamping jaw located above said table and movable toward and away from said table, and fluid pressure means operatively associated with said clamping jaw for moving said jaw and applying gripping pressure to said glass sheet.
 10. Apparatus as defined in claim 9 wherein said fluid pressure means includes control means for applying a relatively high clamping during the drilling operation of said lower drilling tool and a relatively low clamping pressure during the drilling operation of said upper drilling tool. 